Thermally stable photoimaging composition

ABSTRACT

A thermally stable photoimaging composition and a method of using the same, especially on circuit boards as a solder mask is provided. The composition includes a polymerizable resin or resin system, a cationic photoinitiator, a solvent, and an optically transparent ceramic filler. Preferably, the composition has a coefficient of thermal expansion of about 28-40 ppm/°C., which closely matches the coefficient of thermal expansion of the solder used on the circuit board components.

BACKGROUND OF THE INVENTION AND PRIOR ART

This invention relates generally to photoimagable compositions, utilizedin the electronics industry, and more particularly to a cationicallypolymerizable epoxy resin system having photoinitiators which resinsystem has improved heat withstanding properties, improved expansioncoefficient, improved rheological properties and also improvedphotoimaging properties.

There are many different instances where photoimagable compositions areutilized in various industrial processes. In one particular process aphotoimagable composition is utilized as a solder mask and protectivecoating by applying the composition to the underlying printed circuitboard. Thereafter photolithographic techniques are employed to revealvarious underlying structures on the board while masking others so thatsolder may be applied by various solder applying processes to theexposed structures. During the solder applying process the solder willadhere to the exposed underlying components and be prevented fromadhering where the remaining material operates as a solder mask.

The solder mask may also function as a protective coating which protectsthe underlying components from external conditions.

A solder mask can be applied by the appropriate methods; for examplecurtain coating, spray coating, silk screening and dry film. Solder maskmaterial requires certain rheological properties for effective coating.Further, the solder mask must have the properties of providing efficienttransmission of the light or other exposing radiation so as to photolyzethe photoinitiator through whatever thickness of material is applied.The solder mask must demonstrate good adhesion to the substrate circuitboard and the coating must possess a coefficient of thermal expansionsimilar to the surrounding material, such as the solder. The solder maskshould possess good dielectric properties to provide insulation, itshould also provide a good resolution, that is, upon development, theunmasked portions to be soldered should be sharply defined. Also, ofcourse, if the material is to be used as a solder mask, the solder maskmust possess appropriate physical and chemical properties to withstandthe application of the solder material without significant deteriorationor degradation and maintain its coverage over the portions of the boardwherein solder is to be masked. If it is to be used for other purposes,other properties may be required.

Different types of soldering processes require different temperatures toreflow. However, in the process for joining chips to board by the "C4"(controlled, collapse, chip, connection process) in a C4 furnace, thetemperature is typically 360° C. A typical C4 furnace cycle involvesgoing from 300° C. to 360° C., where it remains for about 1 1/2-2minutes before returning to 300° C. However, due to the need for rework,the board may be subject to repeat cycles in the furnace.

In the prior art, frequently a patterned chromium layer was utilized asa solder resist mask for chip joining at 360° C. in a C4 furnace. Thisprocess involves many steps, therefore, is not cost efficient and alsopresents a problem with the disposal of processing waste. Anotherproblem is the inability of the chromium layer, which by nature is verythin, to confine the solder conveniently to prevent the flow intounwanted areas.

There is no known solder mask available which is stable at C4 furnacetemperatures of 360° C. and which also has acceptable resolutionespecially for circuits with line height up to 0.002 inches. Thus, aphoto patternable solder resist mask demonstrating good adhesion to thesubstrates, capable of withstanding 360° temperatures, exhibiting goodresolution while having a coefficient of expansion compatible with theother layers would be beneficial.

There have been many prior art proposals for different photoimagablecompositions including many that use epoxies. Examples of these arefound in the following U.S. Pat. Nos.: 4,279,985; 4,548,890; 4,351,708;4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623;4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197.All of these patents show various resins and photoinitiators for use inphotoimagable compositions many of which are useful as solder masks.However none of them teach or suggest the specific composition of thepresent invention particularly with a controlled coefficient of thermalexpansion and clearly match that of solder.

Another use of a photoimagable dielectric is for building fine lineredistribution layers on circuit boards. While there are photosensitivedielectrics, they have a high coefficient of thermal expansion and/orthey are not capable of withstanding high temperatures (above 360° C.).This factor, combined with high coefficiency of thermal expansion, leadsto cracking and degradation of the redistribution layer.

The present invention, due to its low coefficient of thermal expansion,its ability to withstand high temperatures and its photopatternablecharacteristics, is useful as a fine line redistribution layer.

SUMMARY OF THE INVENTION

The present invention overcomes the drawbacks of the prior art andprovides an improved polymerizable composition comprising a cationicpolymerizable epoxy resin, preferably EPIREZ SU8, a trademark of Hi-TekPolymers in Louisville, Ky., a solvent, preferably a mixture of methylamyl ketone and diisobutyl ketone, an optically transparent ceramicfiller, preferably NOVACUP.sup.(R) L337, silicon dioxide manufactured bythe Malvern Minerals Company, and a photo-sensitive cationic initiator,preferably GE UVE-1014 or Ciba Geigy RD 87-356. The resultingcomposition will polymerize upon exposure to ultraviolet radiation. Byselectively blocking ultraviolet light from the composition, such as byusing a pattern or stencil, the unexposed areas of the composition willnot polymerize. The nonpolymerized portions may be developed, that isremoved, by dissolving such portions in a suitable solvent. Aftercuring, the remaining polymerized portion becomes fully cross linked toa tough protective coat capable of withstanding exposure to temperaturesof 360° C. and having a coefficient of thermal expansion nearly matchingthat of solder.

DETAILED DISCLOSURE OF INVENTION

The composition and preparation of the material useful as a solder maskis disclosed for joining in C4 furnaces, at high temperatures reachingaround 360° C. The invention will provide a photo-patternable process,with specifically defined vias, to confine solder. The invention has acoefficient of thermal expansion of about 28-40 ppm/°C., which iscompatible with the other layers of the package, especially that of thesolder. The invention also exhibits good adhesion to the substrate; thelayer of the solder mask material will act as a protective coat for theentire surface of the electronic surface and act as an encapsulant.

The present invention provides an improved solder resist maskcomprising: an cationic polymerizable epoxy resin, preferably EPIREZSU8, a trademark of Hi-Tek Polymers in Louisville, Ky.; a solvent,preferably a mixture of methyl amyl ketone and diisobutyl ketone, anoptically transparent ceramic filler, preferably NOVACUP.sup.(R) L337, asilicon dioxide manufactured by the Malvern Minerals Company; and aphotosensitive cationic photo-initiator, preferably GE UVE-1014 or CibaGeigy RD 87-356. In the preferred embodiment, the following materialsare present in the following ratios:

    ______________________________________                                        EPIREZ SU8       300 pbw                                                      epoxy resin                                                                   Methyl amyl      100 pbw                                                      ketone                                                                        Diisobutyl        20 pbw                                                      ketone                                                                        NOVACUP          300 pbw                                                      L337                                                                          GE UVE-1014 or    30 pbw                                                      Ciba Geigy RD 87-356                                                          ______________________________________                                    

The above formulation is also used as a photo sensitive dielectric tosequentially build a fine line redistribution layer, one layer at atime, resulting in a multilayer structure.

After a multilayer board is constructed, the solder mask is appliedusing any conventional technique such as screening, dry filmapplication, etc.

The solder mask is then exposed to 50-400 mJ/cm² of ultraviolet light toaccommodate top surface metallurgy. The material is then baked at 60° C.for 15 minutes and developed in a mixture of butyl carbitol and methylethyl ketone, for 2 minutes under ultrasonic agitation. Finally, it iscured at 175° C. for 3 hours. The surface is then mechanically roughenedto promote adhesion. Full additive plating process follows to fabricatetop surface metallurgy.

If required, the above process is repeated to sequentially build morelayers.

THE EPOXY RESIN

While many other cationic polymerizable epoxy resins would be suitable,good results have been obtained using EPIREZ SU8 epoxy resin, anepoxidized octafunctional bisphenol A formaldehyde novolak resinavailable from Hi-Tek Polymers in Louisville, Ky. The EPIREZ resin hasan epoxide value of approximately 4.57 equivalents per kilogram, aweight per epoxide of about 215 and melting point of about 82° C. EPIREZis desirable because of its high functionality. EPIREZ has on an average8 epoxy groups, each of which is reactive; this results in a goodproduct. Cross-linked EPIREZ SU8 also supplies the needed thermostability and photoreactive properties.

The resin may be added in an amount from 30% to 70% by weight of thetotal resin-ceramic filler weight. In the preferred embodiment, theresin is 50% of the total weight of the resin-filler weight.

THE SOLVENT

Many different ketone solvents can be used, including but not limitedto: methyl ethyl ketone, acetone, chloroform, trichloroethylene, andmethylene chloride. However, methyl amyl ketone (MAK) is preferred.Methyl amyl ketone has an appropriate solubility parameter for theEPIREZ SU8 epoxide resin. Methyl amyl ketone is also desirable since dueto its higher boiling point it evaporates more slowly than many of theketone solvents. Methyl amyl ketone is also classified as non-flammablesince its flash point is above 100° F. Furthermore, methyl amyl ketonehas less adverse environmental impact than many of the ketone solventsparticularly in chlorinated solvents. Diisobutyl ketone, which retardsevaporation, also may be added in an amount up to 50% by weight of themethyl amyl ketone.

The solvent may be present from about 10% to 70% by total weight. Below10%, the solid components of the invention may not be soluble, and thematerial may be too thick for application. Conversely, if the solventexceeds 70%, the material may be too thin for proper application.

In the preferred embodiment, the solvent comprised 15.9% by weight oftotal weight.

THE CERAMIC FILLER

Although any optically transparent ceramic filler could be used, NOVACUPL337 available from Malvern Minerals Co. (Arkansas) is preferred becauseof its transparency to ultraviolet light and its complying agenttreatment for promoting adhesion. The phrase "optically transparent" asused herein means that the material transmits in the range used forexposure, which in the preferred embodiment, is ultraviolet light in arange from at least 310 to 400 nm. NOVACUP is silicon dioxide. Thesilicon dioxide is finely divided, having an average particle size ofless than about 6 microns preferably about 3.5 microns. The filler is ofparticular importance because it contributes to the thermal stabilityand thermal expansion of the solder mask. By adjusting the amount of thefiller, the coefficient of thermal expansion of the composition may becontrolled. The ceramic filler may be added from 30%-70% by weight ofthe combined filler--epoxy resin weight. Where the resin exceeds 70%(and the filler is less than 30%), the coefficient of thermal expansionis too high and no longer compatible with the coefficient of thermalexpansion of the solder. Conversely, where the resin is less than the30% (and the filler exceeds 70%), the composition is not cohesive andfalls apart. In the preferred embodiment, the filler is 50% by weight ofthe combined resin filler weight.

THE INITIATOR

While many other cationic photoinitiator are suitable, good results havebeen obtained using Ciba Geigy RD 87-356 initiator, which is iron arenecompound or GE UVE-1014, which is triarylsulfonium hexafluoroantimonatesalt, or more specifically a mixture of a triarylsulfoniumhexafluoroantimonate salt and a tetraryl disulfoniumhexafluoroantimonate salt.

Although the GE UVE-1014 initiator is much faster than the Ciba Geigy RD87-356 initiator, the Ciba Geigy initiator is compatible with additiveelectroless bath.

The initiator may be present from 0.5% to 10%, by weight, of the resin,preferably, 10%. If less than 10% is added, the exposure time must beincreased correspondingly. While a sensitizer could be used to adjustdevelopment of the pattern, it is not necessary.

PREPARATION

The above materials are mixed by preparing a solution of the resin,preferably EPIREZ SU8epoxy resin, in the desired solvent, preferablymethyl amyl ketone. The solution may then be filtered by a conventionalmeans known to one skilled in the art, particularly if a clean productis desired; however, this is not necessary. The solution is then placedin a high speed disperser and the ultraviolet transparent ceramicfiller, preferably NOVACUP L337 is added. The components are mixed belowspeed of about 1,000 rpm for about 5 minutes, or until the silica isincorporated or blended into the solution. The speed is then increasedfrom about 3,000-5,000 rpm until the silica is dispersed in the resinsolution, approximately 10-15 minutes. The initiator, preferably eitherGE UVI-1014 or Ciba Geigy RD 87-356, is added. The material is thenmixed for 5 minutes at speed not greater than 1,000 rpm.

The material may be cast, or it may stored at room temperature, in theabsence of ultraviolet light for at least one month.

If an evaporation retardant such as diisobutyl ketone is to be added, itmay be added at any stage before casting although it is convenient toadd it with the initiator to save an additional mixing step.

The liquid is then coated by conventional methods known to one skilledin the art, such as draw bar, spraying, screening, curtain coating,reverse roll coating or dry film. Good results have been obtained usingdry film depending on the equipment available. Dry film may be preparedby applying with a draw bar or by casting the material in a flat sheeton to Mylar, a trademark of Dupont, (commercially available from Dupontor similar material available from Imperial Chemical Industries) orother polyester material. The material is cast to a desired thickness(usually 0.0005-0.002 inches) and dried for about 30 minutes at 60° C. Apolyethylene sheet is added to the other side and the material is placedon the circuit board. The polyethylene is then removed, and the materialis laminated on to the circuit board, by means known to one skilled inthe art. The material is exposed, through the mylar, to a pattern byusing an ultraviolet light source, such that the material receives from50-400 millijoules/cm². The mylar backing is removed. The material isthen baked for about 15 minutes at 60° C. Then the material is developedin a developing solution, such as a mixture of 70 parts by weight butylcarbitol and 30 parts by weight methyl ethyl ketone, for 2 minutes underultrasonic agitation. The final curing is accomplished at about 175° C.for about 3 hours.

The final protective coat has good resolution and thermal stability atC4 joining temperatures of 360° C. It also has a coefficient of thermalexpansion of about 28-40 ppm/°C. (30-100° C.), which is compatible withsolder which typically has a coefficient of thermal expansion of about27 ppm/°C. In addition, the resulting protective coat demonstrates goodadhesion to substrates, particularly to dielectrics such as fluoropolymer substrates such as Rogers 2800 (manufactured by Rogers Corp,Rogers, Conn.) and cyanate resin dielectrics such as those manufacturedby Hi-Tek Polymers (formerly Interez of Louisville, Ky.).

While several embodiments of the invention have been shown anddescribed, various adaptations and modifications can be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A photo-patternable solder mask thermally stableat 360° C., consisting essentially of a photosensitive cationicphotoinitiator admixed with a cationic polymerizable epoxy resin, asolvent and a filler consisting essentially of an optically transparentsilicon dioxide filler transparent to wavelength of 310 to 400nanometers, said composition having a coefficient of thermal expansionof 28-40 ppm/°C., wherein said solder mask is both an encapsulant and adielectric.
 2. A composition according to claim 1, wherein the epoxyresin is an epoxidized octafunctional bisphenol A formaldehyde novolakresin.
 3. A composition according to claim 2, wherein the epoxy resinhas an epoxide value of approximately 4.57 equivalents per kilogram, aweight per epoxide of about 215 and a melting point of about 82° C.
 4. Acomposition according to claim 1, wherein the epoxy resin is presentfrom about 30-70% by weight of the combined epoxy resin and ceramicfiller weight.
 5. A composition according to claim 1, wherein the epoxyresin is present from about 50% by weight of the combined epoxy resintransparent ceramic filler weight.
 6. A composition according to claim1, wherein the photo-initiator is a triarylsulfoniumhexafluoroantimonate salt.
 7. A composition according to claim 1,wherein the initiator is a mixture of a triarylsulfoniumhexafluorantimonate salt and a tetraryl disulfonium hexafluoroantimonatesalt.
 8. A composition according to claim 1, wherein the initiator isiron arene compound, CIBA GEIGY RD 87-356.
 9. A composition according toclaim 1, which contains from approximately 0.5% to approximately 10%cationic initiator per resin weight.
 10. A composition according toclaim 1, which contains approximately 10% cationic initiator per resinweight.
 11. A composition according to claim 1, wherein the opticallytransparent ceramic filler is silicon dioxide.
 12. A compositionaccording to claim 1, which contains optically transparent ceramicfiller from approximately 30% to approximately 70% by weight of thecombined epoxy resin and ceramic filler weight.
 13. A compositionaccording to claim 1, which contains approximately 50% per resin weightof optically transparent ceramic filler.
 14. A composition according toclaim 1, which contains optically transparent ceramic fillers with amean particle size less than about 6.0 microns.
 15. A compositionaccording to claim 1, wherein the optically transparent ceramic fillerhas a mean particle size of about 3.5 microns.
 16. A compositionaccording to claim 1, wherein the solvent means is an organic solvent.17. A composition according to claim 1, which contains fromapproximately 10% to approximately 70% per solid weight of solventmeans.
 18. A composition according to claim 1, wherein the solvent meansis methyl amyl ketone.
 19. A composition according to claim 1, whereinthe solvent means contains diisobutyl ketone.
 20. A compositionaccording to claim 19, wherein the diisobutyl ketone is present from aneffective amount up to 50% total solvent weight.
 21. A compositionaccording to claim 1, wherein the solvent means is approximately 22%total weight methyl amyl ketone and approximately 2% total weightdiisobutyl ketone.
 22. A circuit board on which is disposed aphotopatterned, polymerized solder mask composition of claim
 1. 23. Acircuit board on which is disposed a photopatterned, polymerizeddielectric composition of claim 1 for fine line redistribution.